To provide focussed vision, an eye must be capable of focusing light on the retina. An eye's ability to focus light on the retina depends, to a large extent, on the shape of the eyeball. If an eyeball is “too long” relative to its “on-axis” focal length (meaning, the focal length along the optical axis of the eye), or if the outside surface (that is, the cornea) of the eye is too curved, the eye will be unable to properly focus distant objects on the retina. Similarly, an eyeball that is “too short” relative to its on-axis focal length, or that has an outside surface which is too flat, will be unable to properly focus near objects on the retina.
An eye that focuses distant objects in front of the retina is referred to as a myopic eye. The resultant condition is referred to as myopia, and is usually correctable with appropriate single-vision lenses. When fitted to a wearer, conventional single-vision lenses correct myopia associated with central vision. Meaning that, conventional single-vision lenses correct myopia associated with vision that uses the fovea and parafovea. Central vision is often referred to as foveal vision.
Although conventional single-vision lenses may correct myopia associated with central vision, it is known that off-axis focal length properties of the eye often differ from the axial and paraxial focal lengths (Ferree et al. 1931, Arch. Ophth. 5, 717-731; Hoogerheide et al. 1971, Ophthalmologica 163, 209-215; Millodot 1981, Am. J. Optom. Physiol. Opt. 58, 691-695). In particular, myopic eyes tend to display less myopia in the retina's peripheral region as compared with its foveal region. This is often referred to as a peripheral hyperopic shift of the image. This difference may be due to a myopic eye having a prolate vitreous chamber shape.
Indeed, a United States study (Mutti et al. 2000, Invest. Ophthalmol. Vis. Sci., 41: 1022-1030) observed that the mean (±standard deviation) relative peripheral refractions at 30° field angle in myopic eyes of children produced +0.80±1.29 D of spherical equivalent.
Interestingly, studies with monkeys have indicated that a defocus in peripheral retina alone, with the fovea staying clear, can cause an elongation of the foveal region (Smith et al. 2005, Invest. Ophthalmol. Vis. Sci. 46: 3965-3972; Smith et al. 2007, Invest. Ophthalmol. Vis. Sci. 48, 3914-3922) and the consequent myopia.
On the other hand, epidemiological studies have shown the presence of correlation between myopia and near work. It is well known that the prevalence of myopia in the well educated population is considerably higher than that for unskilled labourers. Prolonged reading has been suspected of causing a hyperopic foveal blur due to insufficient accommodation. This has led many eye care professionals to prescribing progressive addition or bi-focal lenses for juveniles manifesting progression of myopia. Special progressive lenses have been designed for use by children (U.S. Pat. No. 6,343,861). The therapeutic benefit of these lenses in clinical trials has been shown to be statistically significant in retarding progression of myopia but clinical significance appears to be limited (e.g. Hasebe et al. 2008, Invest. Ophthalmol. Vis. Sci. 49(7), 2781-2789; Yang et al. 2009, Ophthalmic Physiol. Opt. 29(1), 41-48; and Gwiazda et al., 2003, Invest. Ophthalmol. Vis. Sci., Vol. 44, pp. 1492-1500). However, Walker and Mutti (2002), Optom. Vis. Sci., Vol. 79, pp. 424-430, have found that accommodation also increases the relative peripheral refractive error, possibly due to the increased choroidal tension during accommodation pulling the peripheral retina inward.
It is believed that one trigger for myopia progression involves an eye growth signal which compensates for hyperopic defocusing on the peripheral retina, even in circumstances where foveal vision is well corrected.
To correct for both foveal and peripheral vision errors at least two zones of different lens powers are required on the same lens, namely, a central zone or aperture of constant minus power to correct foveal vision, and a peripheral zone of relatively plus power which surrounds the central zone to correct peripheral vision errors. The size of the central zone, the start of the peripheral zone, and the transition between the central zone and the peripheral zone may be varied. For example, the size of the central zone may be adapted according to the typical extent of habitual eye rotation. This may mean, for example, that the central zone may need to have a diameter of between about 10 mm and 20 mm on the lens surface. Typically, 0.5 D to 2.0 D of relatively plus power may be provided in the peripheral zone.
One approach for providing a “power transition” between the central zone of constant minus power and the peripheral zone of relatively plus power involves providing an “instant” transition of the type described in international patent publication WO2007041796. However, such a transition may present undesirable “double vision” type effects to a moving eye.
An alternative approach for providing a power transition between the central zone of constant minus power and the peripheral zone of relatively plus power involves providing a “smooth” aspheric design which introduces a transition or progressive power zone between the central zone and the peripheral zone, as opposed to providing an “instant” power transition. For example, it is known to provide a rotationally symmetric transition zone. However, providing a rotationally symmetric transition zone may introduce a considerable amount of astigmatism which may cause undesirable astigmatic blur on the peripheral retina.
The aspheric single vision lens described in WO2007041796 corrects peripheral hyperopic shift for both distance and near vision. However, hyperopic blur for distance vision typically extends over the entire width of the lens aperture. On the other hand, the hyperopic blur for near vision often extends over a smaller aperture corresponding to the angular size of the near object being viewed, such as a book. Also many near vision tasks, such as reading, demand a far smaller extent of eye rotations than those for many distance vision tasks. Therefore, one would expect that lenses for correcting peripheral hyperopic shift for distance vision would have different requirements than those for correcting peripheral hyperopic shift for near vision, in terms of the size of the central zone and the location and extent of the peripheral zone. One way of addressing the differing requirements is to provide two pairs of lenses, one for the distance vision requirements, the other for near vision requirements. However providing two pairs of lenses is often impractical.
Another approach involves providing an adapted progressive addition lens. A progressive addition lens provides a relatively large upper viewing zone for distance vision tasks, a relatively narrower lower viewing zone having a different surface power from the upper viewing zone to achieve a refracting power corresponding to near vision, and an intermediate zone (or corridor) which extends between the upper viewing zone and the lower viewing zone and provides a power progression there between. In this respect, U.S. Pat. No. 6,343,861 discloses a progressive addition lens having a very short power progression and a relatively large upper and lower viewing zones for viewing distant and near objects respectively.
International patent publication WO2008031166 discloses a progressive addition lens having a relatively plus power in the periphery of the lens which corresponds with the addition power of the lower viewing zone. The lens disclosed in WO2008031166 may introduce a myopic shift on the peripheral retina during distance vision tasks. However, it will not provide effective control of the location of the peripheral image during near vision tasks since the peripheral area of the lower viewing zone, at least in the immediate vicinity of the lower viewing zone, has a lower mean refracting power compared to the central portion of the lower viewing zone and thus does not provide the required relative plus power.
A recent study (Rose et al. 2008, Ophthalmology, Vol. 115, Issue 8, 1279-1285) suggests that juveniles spending more time outdoors, and who, if they are myopes, would mostly experience peripheral hyperopic shift in an unaccommodated eye, show a relatively low tendency of myopia progression. It has been suggested that hyperopic defocus in the periphery of the retina in the presence of positive spherical aberration characterising a normal relaxed eye may not lead to a significant reduction in contrast to trigger the eye growth mechanism. Indeed, measurements and simulations of contrast for different values and signs of defocus for a relaxed eye by Guo et al. (2008), Vision Res. 48, 1804-1811 show the positive (myopic) defocus to be more damaging to contrast on the retina than the hyperopic defocus typically experienced in the peripheral retina by a relaxed myopic eye. It is thought that this is the consequence of interaction between defocus and positive spherical aberration of the relaxed eye. It has been suggested that spherical aberration of the eye may provide a cue for detecting the sign of defocus (Wilson et al. 2002, J. Opt. Soc. Am. A 19(5), 833-839). It is also known that the spherical aberration of the accommodated myopic eye becomes negative (Collins et al. 1995, Vision Res. 35(9), 1157-1163). This would lead to a very different effect of hyperopic defocus on the image contrast in near vision compared to distance vision.
In view of the above, existing ophthalmic spectacle lenses for correcting myopia which provide relatively large central zones of constant power, as proposed in WO 2007041796, may thus fail to remove stimuli for myopia progression for near vision tasks. It would thus be desirable to provide a progressive addition lens which compensates for the peripheral hyperopic shift during near vision tasks while simultaneously providing clear distance vision over a relatively wide aperture field.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of any of the claims